In industrial process measurement technology, especially also in connection with the automation of chemical or other industrial processes, so-called field devices, thus process measuring devices installed near to the process, are employed for producing, on-site, measured-value signals as analog or digital representations of process variables. Examples of such process measuring devices, known per se to those skilled in the art, are described in detail in one or more of the following references from the patent literature: EP-A 984 248, EP-A 1 158 289, U.S. Pat. Nos. 3,878,725, 4,308,754, 4,468,971, 4,524,610, 4,574,328, 4,594,584, 4,617,607, 4,716,770, 4,768,384, 4,850,213, 5,052,230, 5,131,279, 5,231,884, 5,359,881, 5,363,341, 5,469,748, 5,604,685, 5,687,100, 5,796,011, 6,006,609, 6,236,322, 6,352,000, 6,397,683, WO-A 88 02 476, WO-A 88 02 853, WO-A 95 16 897, WO-A 00 36 379, WO-A 00 14 485, WO-A 01 02816 and WO-A 02 086 426.
Examples of the process variables to be registered include a volume flow rate, a mass flow rate, a density, a viscosity, a fill or limit level, a pressure or a temperature, or the like, of a process medium in the form of a liquid, powder, vapor, or gas conducted or available in a corresponding process container, such as e.g. a pipeline or a tank.
For registering the respective process variables, the process measuring device has a corresponding, usually physical-electrical, sensor, which is placed in a wall of the container conducting the process medium or in the course of a process pipeline conducting the process medium, and which serves for producing at least one measurement signal, especially an electrical signal, representing the primarily registered process variable as accurately as possible. For this purpose, the sensor is additionally connected with a suitable measuring device electronics serving especially for a further processing or evaluation of the at least one measurement signal. This includes usually an operating circuit driving the sensor and a measuring and evaluation circuit for further processing of its measurement signals.
Process measurement devices of the described type are usually connected together by way of a data transmission system connected to the measuring device electronics and/or with corresponding process control computers, to which they transmit the measured-value signals e.g. via a (4 mA to 20 mA)-current loop and/or via digital data bus. Serving as data transmission systems in such case are field bus systems, especially serial ones, such as e.g. PROFIBUS-PA, FOUNDATION FIELDBUS, with their corresponding transmission protocols. The transmitted, measured-value signals can be processed further by means of the process control computers and visualized as corresponding measurement results e.g. on monitors and/or transformed into control signals for process adjusting actuators, such as e.g. magnetic valves, electromotors, etc.
For accommodating the measuring device electronics, such process measuring devices include, furthermore, an electronics housing, which, as e.g. proposed in U.S. Pat. No. 6,397,683 or WO-A 00 36 379, can be situated away from the process measuring device and connected therewith only over a flexible cable, or which, as e.g. shown in EP-A 903 651 or EP-A 1 008 836, is arranged directly on the sensor or on a sensor housing separately housing the sensor. Often, the electronics housing then serves, as shown, for example in EP-A 984 248, U.S. Pat. No. 4,594,584, U.S. Pat. No. 4,716,770 or U.S. Pat. No. 6,352,000, also for accommodating some mechanical components of the sensor, such as e.g. membrane, rod, shell or tubular, deformation or vibration bodies, which deform during operation under the influence of mechanical loads; see, in this connection, also the above-mentioned U.S. Pat. No. 6,352,000.
For measuring electrically conductive fluids, flowmeters having an electromagnetic flow sensor are often used. In the following, if expedient, reference will be just to flow sensors, or flowmeters, for short. As is known, electromagnetic flowmeters permit measurement of the volume flow rate of an electrically conducting liquid flowing in a pipeline and represent such measurement in the form of a corresponding, measured value; thus, per definition, the volume of liquid flowing through a pipe cross section per unit time is measured. Construction and manner of operation of electromagnetic flowmeters are known per se to those skilled in the art and are described in detail, for example, in DE-A 43 26 991, EP-A1 275 940, EP-A 12 73 892, EP-A 1 273 891, EP-A 814 324, EP-A 770 855, EP-A 521 169, U.S. Pat. Nos. 6,031,740, 5,487,310, 5,210,496, 4,410,926, 2002/0117009 or WO-A 01/90702.
Flow sensors of the described type usually each exhibit a non-ferromagnetic, measuring tube which is connected into the pipeline in a liquid-tight manner, for example by means of flanges or threaded joints. The portion of the measuring tube which contacts the liquid is generally electrically non-conductive, so that no short circuit is present for a voltage induced in the liquid according to Faraday's law of electromagnetic induction by a magnetic field cutting across the measuring tube.
In keeping with this practice, metal measuring tubes are commonly provided internally with a nonconductive lining, e.g., a lining of hard rubber, polyfluoroethylene, etc., and are themselves generally non-ferromagnetic; in the case of measuring tubes made entirely of plastic or ceramic, particularly of alumina ceramic, the nonconductive lining is, in contrast, not necessary.
The magnetic field is produced by means of two coil assemblies, each of which is, in the most frequent case, mounted on the outside of the measuring tube along a diameter of the latter. Each of the coil assemblies commonly includes an air-core coil or a coil with a core of soft magnetic material.
To ensure that the magnetic field produced by the coils is as homogeneous as possible, the coils are, in the most frequent and simplest case, identical and electrically connected in series, thus aiding one another, so that in operation they can be traversed by the same excitation current. It is also known, however, to pass an excitation current through the coils alternatingly in the same direction and in opposite directions so as to be able to determine, for example, the viscosity of liquids and/or a degree of turbulence of the flow; see, in this connection, also EP-A 1 275 940, EP-A 770 855, or DE-A 43 26 991.
The excitation current just mentioned is produced by an operating electronics; the current is regulated at a constant value of, e.g., 85 mA, and its direction is periodically reversed. The current reversal is achieved by incorporating the coils in a so-called T network or a so-called H network; for the current regulation and current reversal, see U.S. Pat. No. 4,410,926 or U.S. Pat. No. 6,031,740.
The mentioned, induced voltage appears between at least two galvanic (thus, wetted by the liquid), measuring electrodes or between at least two capacitive (thus, arranged within the wall of the measuring tube), measuring electrodes, with each of the electrodes picking up a separate potential.
In the most frequent case, the electrodes are mounted at diametrically opposed positions such that their common diameter is perpendicular to the direction of the magnetic field, and thus perpendicular to the diameter on which the coil assemblies are located. The induced voltage is amplified, and the amplified voltage is conditioned by means of an evaluation circuit to obtain a measurement signal which is recorded, indicated, or further processed. Suitable evaluation electronics are familiar to those skilled in the art, for example from EP-A 814 324, EP-A 521 169, or WO-A 01/90702.
In principle, the absolute value of the potential at the respective electrode is of no significance for the measurement of the volumetric flow rate, but only on condition that, on the one hand, the potentials lie in the dynamic range of a differential amplifier following the electrodes, i.e., that this amplifier must not be overdriven by the potentials, and that, on the other hand, the frequency of potential changes differs significantly from the frequency of the above-mentioned current direction reversal.
The potential at each electrode is not only dependent on the magnetic field according to Faraday's law—the geometrical/spatial dimensions of the measuring tube and the properties of the liquid enter into this dependence—, but this measurement signal, which is based on Faraday's law and should be as clean as possible, has interfering potentials of different geneses superimposed on it, as already discussed in EP-A 1 273 892 or EP-A 1 273 891. These interfering potentials can contribute substantially to a degradation of the measurement accuracy.
A first kind of interfering potential results from inductive and/or capacitive interference which emanates from the coil assemblies and their leads, and which changes the electric charge on the capacitor that exists at the boundary layer between electrode and liquid. As a result of asymmetries in the concrete structure of the flow sensor, particularly as far as the conductor routing to the coil assemblies and to the measuring electrodes is concerned, the interfering potential of one electrode generally differs from the interfering potential of the other electrode.
This—first—effect may, on the one hand, restrict the dynamics of the differential amplifier. On the other hand, the value of the difference between the interfering potentials of the electrodes is subject to variances in flow-sensor parameters due to manufacturing tolerances. Also, the determinable dependence of the electrode potentials on the velocity of the liquid is partly due to this effect, because at low velocities, the above-mentioned charges at the boundary layer between electrode and liquid are not removed by the latter.
A second kind of interfering potential is caused by particles of foreign matter or by air bubbles which are entrained by the liquid and which, when colliding with an electrode, cause sudden changes in the potential of the electrode. The decay time of these changes is dependent on the type of liquid and is generally greater than the rise time of the changes.
This—second—effect, too, results in an erroneous measurement signal. The error is also dependent on the potential of the electrode. Since this potential varies from flow sensor to flow sensor due to manufacturing tolerances, as was explained above, the second effect adds to the first effect, so that the individual flow sensor units differ widely in their behaviors, this being, of course, highly undesirable.
A third kind of interfering potential is caused by coatings deposited by the liquid on the measuring electrodes, as is also described in U.S. Pat. No. 5,210,496, for example. The formation of the coatings is very strongly dependent on the velocity of the liquid. The differences in the behavior of the individual flow sensor units may be further increased by the formation of the coatings.
EP-A 1 273 892 proposes a method of operating an electromagnetic flow sensor in which the development of the above-mentioned interfering potentials of whatever kind is prevented, or at least their effect is significantly reduced, by at least intermittently applying voltage pulses generated by means of the evaluation and operating circuit to at least one of the two measuring electrodes. The use of this method can lead to a considerable improvement in the accuracy of electromagnetic flowmeters, particularly in the case of single-phase or thoroughly mixed multiphase liquids. Beyond this, in EP-A 337 292 or WO-A 03/004977, for example, methods are described in which the measuring electrodes, particularly by being short-circuited to ground in timed sequence or by application of a harmonic alternating voltage, are subjected over a prolonged period of time to an interfering-potential-eliminating discharge voltage.
One disadvantage of this prior-art method of measurement, and of flow sensors using this method, is that in the case of multiphase liquids with distinctly separated liquid phases or in the case of pasty-viscous liquids, for example, a rather stochastic, practically inestimable distribution of the entrained particles of foreign matter or of the gas bubbles is to be expected, which can hardly be calibrated. To a corresponding extent, at least interfering potentials of the second kind cannot be sufficiently reliably removed from the measuring electrodes.